1. Field of the Invention
The present invention relates to a telephone system, and in particular, to a circuit for sending out a ringing signal to a telephone set through a telephone line.
2. Description of the Prior Art
The ringing signal comprises an A.C. signal having a low frequency such as 16 Hz and a comparatively high effective voltage such as 75V and being sent out repeatedly for a predetermined time period such as one second with a pause interval such as two seconds.
In the prior art, an oscillator is used for generating the ringing signal and has a comparatively large size because vacuum tubes, a large-sized output transformer and other components are required to generate the ringing signal of such a high voltage and a low frequency. A relay is used to send out the oscillator's output to the telephone line repeatedly with the pause intervals. Spark unadvantageously generates at the relay contacts opening and closing.